A vernier-type linear motor has been proposed, in which a mover period and a stator period are different from each other to improve driving force (torque) of the linear motor. For example, a non-patent literature 1 discloses a PM-type linear vernier motor. This non-patent literature 1 proposes a technique which reduces pulsing force caused by cogging force and end effect occurring in the linear vernier motor.
Also, a patent literature 1 discloses an electric power steering apparatus using the linear vernier motor. This electric power steering apparatus includes: a pinion which is provided at a tip of a steering shaft and rotates with the steering shaft; a rack bar which engages with the pinion and makes a steering angle change by transforming rotary motion of the steering shaft into rectilinear motion of a longitudinal direction; a housing which houses the pinion and the rack bar; an electric motor which gives auxiliary power to the rack bar. The electric motor is a linear-type vernier motor characterized in the followings. In the housing, fixation slots, in which the number of the fixation slots is a first number in the longitudinal direction, are respectively provided away from each other by a first gap. In addition, movable slots, in which the number of the movable slots is a third number, are provided away from each other by a second gap within an addition length of whole fixation slots and the same number first gaps in the longitudinal direction of the rack bar, the third number being less than or more than the first number by a second number. Furthermore, permanent magnets are housed inside each of the fixation slots and each of the movable slots. Moreover, three phase AC coils, in which the second number is the number of magnetic pole pairs, are housed in one group of each of the fixation slots and each of the movable slots.
As a related technique, Japanese Patent Publication JP 2010-114980 A discloses a linear actuator. This linear actuator includes: a coil portion configured to include a plurality of coils respectively applied with AC currents having different phases from one another; and a shaft portion configured to pass through an inside of the plurality of coils. The shaft portion includes: a plurality of permanent magnets configured to be arranged along a central axis such that opposite magnetization directions face to each other in a direction of the central axis; and a plurality of intermediate members, each configured to be arranged between adjacent two of the plurality of permanent magnets. A saturation magnetic flux density of each of the plurality of intermediate members is higher than a saturation magnetic flux density of each of the plurality of permanent magnets. Or, the plurality of intermediate members includes permanent magnets which are arranged along the central axis such that magnetization directions are perpendicularly outward and perpendicularly inward with respect to the direction of the central axis alternately. Further, combining members may be arranged around each of the permanent magnets, the combining members having a saturation magnetic flux density higher than a saturation magnetic flux density of each of the plurality of permanent magnets. Each of the plurality of intermediate members may have a thickness in the central axis direction, the thickness increasing with increasing a distance from the central axis. In this linear actuator, driving force is improved by canceling magnetic flux saturation on the mover side.